Flat cables, which not only have the smallest possible dimensions and high permanent flexibility, but also permit transmission of very high data rates with minimal transit time differences, for example, in the range of 2.5 Gbit/s, are required for certain applications. Such applications include mobile telephones, PDAs (personal digital system) or small computers called palmtops and laptops, which have parts that can be tilted and/or rotated relative to each other, between which high-speed data transmission is required. Because of the small dimensions, especially in the case of mobile telephones and PDAs, such data connections must be produced via flat cables with the smallest possible dimensions, even micro flat cables.
Particularly reliable data transmission is obtained with so-called differential signal transmission, in which the data pulses being transmitted are transmitted via two signal conductors, in non-negated form via one of the two signal conductors and in negated form via the other signal conductor. A specific data bit is therefore transmitted on one of the two signal conductors with high potential and, at the same time, on the other of the two signal conductors with low potential, in which case descending flanks occur on one of the two signal conductors during rising flanks on the other of the two signal conductors and vice versa. This differential signal transmission, with opposite pulse shape over the two signal conductors, permits particularly reliable data transmission. Common-mode disturbances, like crosstalk, are filtered out by the differential signal transmission and disturbances from radiation and emission are significantly reduced.
A cable having very high uniformity with respect to impedance and surge impedance is required for high-speed data transmission. In a flat cable, this means that electrical conductors adjacent to each other, separated by a dielectric, which form a signal conductor pair, must have a spacing from each other that not only must be very well defined, but also must have high-grade uniformity. This uniformity must not only be ensured over the entire length of the cable, but also during operation of the cable, during which bending, twisting and/or flexing movements of the cable must not lead to a change in impedance.
In the context of the present disclosure, the term adjacent is understood to mean proximity in the flat cable thickness direction and/or in the flat cable width direction.
The electrical parameters required for electrical cables that must be suitable for high-speed data transmission are determined quite essentially by the spacing between the two signal conductors, apart from the material of the dielectrics separating the two signal conductors. This is particularly true for the impedance or surge impedance. Ordinary flat cables are one-layered, i.e., all their electrical conductors are situated in the same plane. Common examples of this are shown in EP 1 271 563 A1, EP 0 961 298 B1 and EP 0 903 757 B1. In all these known flat cables, the electrical conductors are embedded between two insulation sheets corresponding to the width of the flat cable, in which shielding is additionally provided in the case of EP 0 903 757 B1, formed by two electrically conducting layers that enclose the outsides of the two insulation sheets. These cables are suitable only for low frequencies and, in the case of a shielded version, the flexibility and packing density necessary for the applications mentioned in the introduction cannot be reached. The unshielded versions are often not satisfactory with respect to EMC (electromagnetic compatibility) either.
Alternative solutions, like shielded flexible circuit boards and shielded one-layered flat cables, do not satisfy the typical mechanical flex-lifetime requirements of several hundred thousand flex cycles, as are common in the devices mentioned in the introduction with parts that are movable relative to each other.
With the usual methods and equipment for the production of flat cables, it is not possible to ensure a spacing between the electrical conductors lying next to each other in the flat cable width direction with as high a uniformity as would be required for uniformity of impedance of a flat cable suitable for high-speed data transmission.